In the three-dimensional integration of semiconductor chips, a number of levels of thinned chips are adjusted on one another and electrically connected. Component placement takes place in an automatic placement machine in which the individually separated chips are placed in an adjusted manner onto a wafer. The surfaces of the wafer and of the chips have surfaces that are structured with contact areas in mirror-image patterns in relation to one another, so that the contacts belonging to one another can be applied with great accuracy such that they fit on one another. The surface of the wafer consists for example of bare copper, that of the chips for example of tin or a low-melting tin alloy. In the case of this process, the problem arises that the bare metal surfaces, especially the copper surface, oxidize because of the high process temperatures. An oxidation layer, however, prevents reliable contacting.
Soldering methods for the soft-soldering of copper surfaces with lead-tin-based solders are known and work very reliably. In these cases, the thicknesses of the solder layers lie in the range of 100 μm. During the heating operation, the surfaces are flushed with forming gas, the reactive fraction of which prevents oxidation. A soldering technique which is known by the terms “isothermal solidification” or SOLID uses extremely thin solder layers of less than 5 μm in thickness. In the case of this soldering method, complete alloying of the solder metal with the adjoining metal surfaces occurs during the soldering operation, accompanied by the formation of intermetallic phases, whereby the melting point rises and the solder melt isothermally solidifies. While thin oxide layers may break up when a thicker solder layer is used, so that adequately good contacting can be established, even a thin oxide layer is enough to completely prevent wetting of the metal surface with a very thin layer of the solder material.
An especially important requirement in the case of the SOLID soldering method is that the metal surfaces which are being soldered to one another remain completely free of oxide. Therefore, it is necessary to achieve a situation in which as little atmospheric oxygen as possible gets into the atmosphere of forming gas surrounding the wafer during the heating operation. For this purpose, the forming gas must enclose the chip region to be heated as impenetrably as possible. However, this cannot take place completely, since both surfaces must be freely accessible during the adjustment.
GB 2244374 A describes a device for soldering contacts on vertically integrated semiconductor chips in which a clamping device, a flushing device, a chip mount and a heater are provided. DE 198 50 595 A1 and DE 195 04 967 A1 disclose methods of producing chip-carrier connections in which the solder points are heated from the rear side of the chip.
DE 197 47 846 A1 disclosed a method of contacting components, in particular for connecting a semiconductor chip to a leadframe, in which isothermal solidification is used. A silicon chip can be connected to a silicon substrate in the way described. The connection of the contacts applied to the subelements takes place by means of an intermediate layer, which is melted by supplying vibrational energy, and the connection is established by isothermal solidification. Thermal support for this operation may be provided, since the reaction is expediently carried out at elevated temperature under inert gas.